When operating in a downhole well, tools used for the operation may not be submergible themselves. Some tools are positioned at the front of coiled tubing and are driven forward by pushing the tubing further down the well. Other tools are lowered into the well by means of a wireline, and gravity will thus ensure that the tool submerges. Hence, not all tools are capable of moving in the well and thus need to be moved forward in the well by an additional tool. In particular, this is the case in the horizontal part of the well, as gravity cannot aid in the movement.
Several tools have been developed for this purpose, inter alia one running on a caterpillar track. However, this tool has the disadvantage that it cannot always hold its footing in the more uneven parts of the well, and in some cases it is impossible for such a tool to pass a place where two well pipes meet but do not abut hence leaving a gap. Another tool has wheels driven by means of a roller chain and all driven by one motor. However, if the motor is unable to drive all wheels, the tool is unable to drive itself any further. This may be the case if the well has an obstacle and one wheel is unable to be driven across the obstacle.